Hi
Personally I am not using IGTV but you can set up VP to preview and export the 9:16 format although the timeline will still show the clips in a 16:9 thumbnail.
First go to Tools/Options/Display/ and change the values of width and height to an aspect ration of 9:16. (Or use the Custom) option) The resolution chosen can affect the speed of caching etc. so pick some lowish values for a start e.g. 225 x 400 You can change them later.
The Preview display will now come up with the 9:16 AR but the Clip Bin and Timeline will still be 16:9 with the chequered border (or black bars) to each side but the area of the image is what you have set. Editing is no different to before except you will now be working within the preview area.

When exporting simply click the Auto detect option the the export window or use the Custom settings obtained from the Resolution button menu to enter your chosen values.If you don't auto detect then the custom values should be those of the original clips.
Pretty certain others might contribute to this thread if they have found other options.
Nat